There are conventionally known light irradiation apparatuses in which laser light incident from a light source part is divided into a plurality of light fluxes by a plurality of cylindrical lenses in a cylindrical lens array, and irradiation regions of the light fluxes are superimposed on one another on an irradiation plane by another lens. With such light irradiation apparatuses, interference fringes (speckle patterns) appear on the irradiation plane due to interference between the light fluxes. In the case of using a bar laser having a plurality of light emitting points (emitters) in the light source part, the coherence is not high among the plurality of light emitting points. However, the same interference-fringe problem occurs because laser light emitted from a single light emitting point is divided into a plurality of light fluxes and the light fluxes are superimposed on one another.
Thus, techniques that provide an optical path length difference generation part in the vicinity of the cylindrical lens array have been proposed (see, for example, Japanese Patent Application Laid-Open Nos. 2002-321081 and 2009-94329). The optical path length difference generation part includes a plurality of transparent parts that produce a difference in optical path length between them, the difference being longer than the coherence length (coherence distance) of the laser light, and light fluxes that have passed through the transparent parts or light fluxes that travel toward the transparent parts respectively enters the cylindrical lenses. This suppresses variations in light intensity due to interference between the light fluxes on the irradiation plane.
The provision of the above optical path length difference generation part, however, increases the size of the light irradiation apparatus because the plurality of transparent parts need to produce a difference in optical path length longer than the coherence length of the laser light, thus increasing the lengths of the transparent parts. There is thus demand for techniques that suppress variations in light intensity due to interference between the light fluxes on the irradiation plane while suppressing an increase in the size of the light irradiation apparatus.